Viruses are acellular microorganisms, meaning they are not composed of cells. A virus fundamentally consists of proteins and genetic material (either DNA or RNA, but never both) and remains inert outside of a host organism. To multiply and infect other hosts, viruses must incorporate themselves into a host cell and co-opt its cellular mechanisms. They can infect all types of cells, from human cells to other microorganisms, and while some are responsible for diseases like the common cold or Ebola, many do not cause disease.